1. Field of the Invention
The present invention relates to a method of and apparatus for transporting lead frames stacked on a magazine to a transport path one by one.
2. Description of the Related Art
Lead frames for use in the assembly of electronic components are stacked on a magazine and are transported to a transport path one by one by means of a transport head. Subsequently, the lead frame is transported to a line for assembling electronic components, such as a die bonder, along the transport path. In this case, the lead frames are stacked on the magazine in two ways, namely, a way in which lead frames are directly stacked on the magazine (this form will be herein referred to as a "direct stacking mode"), and a way in which lead frames are stacked while inter leaves are sandwiched, respectively, between them (this form will be herein referred to as an "inter-leaf stacking mode").
FIG. 1 is a side view showing an existing apparatus for transporting lead frames. In this example, lead frames 1 are stacked by the inter-leaf stacking mode; that is, lead frames 1 and inter leaves 2 are alternately stacked in a magazine 3 such that the inter leaves 2 are sandwiched, respectively, between the lead frames 1. An elevation plate 4 is provided in the magazine 3, and the lead frames 1 and the inter leaves 2 are stacked on the elevation plate 4 which is actuated so as to be raised or lowered by means of drive means (not shown). As a result of the elevation plate 4 being gradually elevated by an elevation rod 5, the upper surface of the topmost lead frame 1 is maintained at a given level.
A transport path 6 for use with the lead frame 1 is disposed besides the magazine 3. Two-sheet pick-up sensors 8a, 8b are disposed between the magazine 3 and the transport path 6. A recovery section 7 for recovering the inter leaves 2 is disposed on the side of the magazine 3 opposite to the side thereof facing the transport path 6. A transport head 10 has nozzles 11 for vacuum-chucking the lead frame 1, and each of the nozzles 11 has a suction pad 12 at the lower end. A recovery head 13 has nozzles 14 and suction pads 15, both of which vacuum-chuck the inter leaf 2. The transport head 10 and the recovery head 13 are integrally joined together by means of a frame 16 and are moved in vertical and horizontal directions by means of unillustrated means.
The operation of the foregoing existing lead frame transport apparatus will now be described. First, an explanation will describe the lead frames 1 stacked by the inter-leaf stacking mode; that is, a case where the inter leaves 2 are sandwiched, respectively, between the lead frames 1. The transport head 10 is raised or lowered in the space above the magazine 3 to thereby vacuum-chuck the topmost lead frame 1 stored within the magazine 3. Prior to picking up the topmost lead frame 1 from the magazine 3 by vacuum chucking, the inter leaf 2 laid on the topmost lead frame 1 is picked up by the recovery head 13 and is recovered by the recovery section 7.
Next, the transport head 10 horizontally travels to the position immediately above the transport path 6 while maintaining the picked-up lead frame 1 and is lowered at the position, thereby loading the lead frame 1 on the transport path 6. On its way to the transport path 6, the lead frame 1 is subjected to a determination, by the two-sheet pick-up sensors 8a, 8b, as to whether or not two lead frames 1 are erroneously picked up. The lead frame 1 loaded on the transport path 6 is sent to a line for assembling electronic components, such as a die bonder.
Simultaneously with the transport head 10 being lowered to load the lead frame 1 on the transport 6, the recovery head 13 is lowered to the magazine 13, thereby vacuum-chucking the inter leaf 2. The recovery head 13 travels to the position immediately above the recovery section 7 while vacuum-chucking the inter leaf 2. Simultaneously with the transport head 10 being lowered to vacuum-chuck the next lead frame 1, the recovery head 13 releases the inter leaf 2 from a vacuum-chucked state, so that the recovered inter leaf 2 is placed on the recovery section 7.
Next, an explanation will describe lead frames stacked by the direct stacking mode. In this case, since there are no inter leaves 2 to be recovered by the recovery head 13, the recovery head 13 is in an idle state in which it does not perform a vacuum-chucking operation. Only the transport head 10 performs operations in the same manner as does in the foregoing case of the lead frames stacked by the inter-leaf stacking mode.
The lead frame 1 is manufactured by processing metal into a thin metal plate having light weight and a smooth surface. For this reason, when the lead frame 1 is picked up from a stack of lead frames-which are directly stacked in close contact with one another within the magazine 3-by vacuum chucking, so-called "two-sheet pick-up error"; that is, two frame leads instead of one being erroneously picked up by one operation, will occur with certain frequency. This error can be attributed to several reasons; that is, adhesion strength originating from the aforementioned surface smoothness, or minute burrs formed along the edges of the lead frames 1 meshing with one another. A technique for preventing such a two-sheet pick-up error has not yet been established. Therefore, in order to prevent two lead frames 1 from being transported, the lead frame transport apparatus performs the detection of two lead frames in the course of the lead frame 1 being transported to the transport path 6 from the magazine 3.
Since the two-sheet pick-up error does not occur with high frequency, it is not desirable to provide the lead frame transport apparatus with a recovery mechanism to recover the lead frames 1 in the event of two lead frames being picked up, from the viewpoint of cost. For this reason, when the two-sheet pick-up error sensors detect the picking up of two lead frames, an alarm is issued simultaneously with the lead frame transport apparatus being deactivated. The lead frame apparatus has the disadvantage of requiring restart of the lead frame transport apparatus after the operator has taken necessary steps, such as steps to manually recover the two lead frames 1.